Optical PM-fibers are used for example for sensors, where the polarisation state of light used when communicating information through the optical fibers must be strictly controlled. Commercially available polarisation maintaining fibers are constructed like ordinary optical fibers comprising a centrally located core and a surrounding sleeve or cladding having a cylindrical exterior surface. Moreover, in the cladding, as seen in a cross sectional view, in a common kind of PM-fibers, two separate, substantially identically regions of highly doped glass, "stress zones" are provided, this structure resulting in that a PM-fiber is not rotationally symmetric in its physical structure nor in its optical structure. When connecting such PM-fibers to each other it is naturally important that the asymmetric regions such as said stress zones of the two ends of the fibers are placed opposite each other before the very splicing process, so that the polarisation axes of the fibers are aligned with each other and will remain so after the splicing. Other kinds of optical PM-fibers have other types of axial asymmetries, but in the splicing the asymmetries should generally be aligned with each other. Therefor a controlled rotation is required of at least one of the fibers immediately before or in the splicing process.
U.S. Pat. No. 4,986,843, U.S. Pat. No. 5,147,434, U.S. Pat. No. 5,149,350, and U.S. Pat. No. 5,156,663 all disclose welding devices for splicing optical PM-fibers, the welding devices comprising rotation means for adjusting the angular position of the fibers before the splicing process. The devices comprise, as seen from the exterior, far end of the fibers, in a direction towards the splicing position, rotation devices 44A and 44B having support devices 62 comprising V-grooves 66 and clamping devices 61 at their interior, proximate ends, support blocks 24 comprising clamping devices 26 located adjacent thereto for clamping fiber portions having remaining protective coatings and retaining devices 22 and support devices 20 comprising V-grooves for the naked fiber portions most adjacent to the splicing position and welding electrodes 38 and optical devices for viewing/imaging the fiber ends at the splicing position and for aligning the fiber ends. The first rotation device 44A is intended to be rotated manually by means of a slotted knob 48. The rotation device 44A comprises in addition to said knob a platform 42 and a retaining means 61, see FIG. 7. The second rotation device 44B is identical to the first one except that it is provided with a slotted gear pinion instead of a knob and is intended to be rotated by means of an electric motor 72 through a gear transmission 74, 76. This construction is complicated, has in addition a large extension in the longitudinal direction of the fibers and three support parts or retaining/clamping devices on each side of the splicing position.
In the article "A simple technique for high accuracy core-cladding concentricity measurement of single mode fibers", D N Ridgway, L. J. Freeman, Technical digest--Symposium on optical fiber measurements 1982, pp. 139-142, a simple and reliable technique is described for determining the concentricity of the core of an optical fiber in relation to its cladding. A test measurement equipment is shown in FIG. 2 consisting of a manual rotation device, a positioning device and a vacuum chuck.